1. Field of the Invention
The present invention relates to a device for detecting obstacles around a vehicle by using reflection beams. More particularly, the invention relates to a device for detecting obstacles, for use in vehicles, which is capable of correctly detecting the positions and sizes of obstacles without driving up the cost.
2. Prior Art
Various devices have heretofore been proposed for detecting obstacles around a vehicle by using reflection beams as taught in, for example, Japanese Unexamined Patent Publications (Kokai) Nos. 296777/1993, 92263/1995 and 254887/1989.
According to Japanese Unexamined Patent Publication (Kokai) No. 296777/1993 which is concerned with a device for guiding a vehicle to a target position, a scanning-type laser distance sensor emits a laser beam in a predetermined direction and measures the time until the reception level of the reflection beam thereof exceeds a predetermined value, thereby to detect the distance to the obstacle.
When the laser beam is used as an emitted beam, however, it has been known that a reliable detection performance is not obtained though the resolution is high concerning the angular position of the obstacle.
For example, when the obstacle comprises a material that little reflects the laser beam, the reception level of the reflection beam may become smaller than a threshold value, and is not detected.
If the threshold value is lowered to enhance the detection performance, noise may be erroneously picked up. Thus, limitation is imposed on lowering the threshold value.
According to Japanese Unexamined Patent Publication (Kokai) No. 92263/1995 which is concerned with a device for detecting obstacles, for use in vehicles, on the other hand, an ultrasonic wave distance sensor emits ultrasonic waves and measures the time until the reception level of the reflection wave thereof exceeds a predetermined threshold value, thereby to detect the distance to the obstacle.
When the ultrasonic waves are used as an emitted beam, it has been known that the resolution is low concerning the directivity though the device can be relatively cheaply constituted and the distance to the obstacle can be highly reliably detected.
Even when the ultrasonic waves are used, the detection performance decreases when the obstacle comprises a material that little reflects ultrasonic waves.
As described above, the conventional devices for detecting obstacles, for use in vehicles have problems in that performance for detecting obstacles decreases when it is attempted to improve the resolution, and the detection performance is adversely affected by noise resulting in an erroneous detection when it is attempted to improve performance for detecting obstacles, no matter when ultrasonic waves, electromagnetic waves or laser beams are used.